The present invention relates to transducers that employ resonant beams for measuring acceleration, pressure, temperature and other variables based on induced strain along the beam, and more particularly to the use of dielectrically isolated circuit elements formed on such beams, for oscillating the beams and sensing resonant frequencies.
Resonant sensors are well known as a means to obtain highly accurate measurements. Vibrating transducers have been used in precision accelerometers and pressure sensors. These sensors operate on the principle that the natural frequency of vibration (i.e. the resonant frequency of an oscillating beam) is a function of the induced strain along the beam. Tensile forces tending to elongate the beam increase its resonant frequency, while forces tending to compress or otherwise shorten the beam reduce its natural frequency. The frequency output of resonant gauges is readily converted to a digital reading reflecting the measured parameter, e.g. using a counter and a reference clock. Accordingly, such devices are simple and reliable, providing high discrimination while using a relatively simple interface to digital signal processing circuitry.
U.S. Pat. No. 5,090,254 (Guckel et al) discloses a resonant beam transducer including a polysilicon beam mounted to a substrate for vibration relative to the substrate, and a polysilicon shell surrounding the beam and affixed to the substrate to form a cavity which is sealed and evacuated. The beam is oscillated by supplying an oscillating voltage to an electrode on the shell.
One particularly effective device of this type is a resonant integrated microbeam sensor disclosed in U.S. patent application Ser. No. 07/937,068, filed Aug. 31, 1992, now U.S. Pat. No. 5,275,055, and entitled "RESONANT GAUGE WITH MICROBEAM DRIVEN IN CONSTANT ELECTRIC FIELD", assigned to the assignee of this application. The vibrating transducer is an elongate polysilicon flexure beam attached at both ends to a silicon substrate, and enclosed within a vacuum chamber formed by the substrate and a rigid polysilicon cover. A pair of bias electrodes on opposite sides of the beam, one formed in the substrate and the other formed in the rigid cover, cooperate to provide a constant electrical field about the flexure beam. A drive electrode on the flexure beam is selectively charged to cause the beam to oscillate. A piezoresistive element also is formed on the flexure beam, to sense beam position and generate a beam position signal used to control the drive oscillator. Thus, the beam tends to oscillate at its natural resonant frequency.
The sensor can be mounted to position the flexure member along a pressure responsive flexible diaphragm or along a beam of an accelerometer. So positioned, the beam is alternatively elongated and shortened as the diaphragm or beam fluctuates in response to pressure differentials and accelerations, respectively. The drive electrode and piezoresistor can be formed by boron ion implantation into the polysilicon flexure beam.
While this sensor is effective and accurate in numerous applications, it is subject to parasitic coupling from current leakage due to a parasitic capacitance between the drive electrode and piezoresistor, and between these components and the bias electrodes. The difficulty increases with smaller sized flexure beams and correspondingly reduced space available to physically separate the piezoresistor and drive electrode. One approach to counteracting parasitic coupling is shown in FIG. 10 of the aforementioned '068 patent application. A shield electrode 136 is implanted into the flexure beam, between the drive electrode and the piezoresistor. Of course, this requires additional space on the flexure beam.
Several difficulties arise from forming the piezoresistor and drive electrode by ion implantation. Due to lateral diffusion, it is difficult to precisely and repeatably determine the size and impedance of the piezoresistors and drive electrodes. Thermal transient effects limit the effectiveness of the transducers at high temperatures. Finally, the drive electrode and piezoresistor are exposed to potential degradation due to impurities or ionic contamination.
Therefore, it is an object of the present invention to provide a resonant beam sensing device in which drive electrodes and sense electrodes on the resonant beam are dielectrically isolated from one another to virtually eliminate parasitic resistive coupling between them, even at high temperatures, without the need for a shield electrode.
Another object of the invention is to provide an electrostatically driven resonant microbeam in which drive circuit element and position sensing circuit element on the beam can be effectively isolated from the beam itself.
A further object is to provide an electrostatically oscillated integrated resonant microbeam sensor capable of operation at high temperatures.
Yet another object is to provide a process for fabricating integrated resonant microbeam sensors in a manner that more precisely replicates the selected impedances of piezoresistors and drive elements formed on the flexure beams.